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doi:10.1136/bmj.c7023

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Annemieke C C Evers, PhD student, resident in gynaecology1,
Hens A A Brouwers, neonatologist1,
Chantal W P M Hukkelhoven, epidemiologist2,
Peter G J Nikkels, pathologist1,
Janine Boon, gynaecologist3,
Anneke van Egmond-Linden, gynaecologist4,
Jacqueline Hillegersberg, paediatrician5,
Yvette S Snuif, gynaecologist6,
Sietske Sterken-Hooisma, midwife secondary care7,
Hein W Bruinse, professor of obstetrics1,
Anneke Kwee, gynaecologist1

¹University Medical Center Utrecht, 3584 EA Utrecht, Netherlands
²Netherlands Perinatal Registry, Utrecht, Netherlands
³Department of Obstetrics and Gynaecology, Diakonessenhospital, Utrecht, Netherlands
⁴Department of Obstetrics and Gynaecology, Zuwe Hofpoort Hospital, Woerden, Netherlands
⁵Department of Pediatrics, St Antonius Hospital, Nieuwegein, Netherlands
⁶Department of Obstetrics and Gynaecology, Admiraal de Ruyter Hospital, Goes, Netherlands,
⁷Department of Obstetrics and Gynaecology, Meander Medical Center, Amersfoort, Netherlands

a.evers{at}umcutrecht.nl

De Jonge and colleagues and Pop and Wijnen commented on the nomenclature of our study.1 2 We performed a cohort study, including cases in a prospective manner. Thus important problems associated with retrospective data collection—such as dependence of the investigator on the availability and accuracy of medical case records and classification bias of the cases—are prevented. The population at risk was estimated retrospectively …